The fitting I linked you to previously, does not open the valve, rather the incoming pressure must overcome the spring on the schrader. This means that when you stop pumping and go to remove the fitting, the schrader is already closed and won't leak.

The downside, is that you need to generate the extra 70psi to crack the schrader open.

Of course! Hand hitting head.

All one needs to do is to thread on a fitting onto the Schrader that seals to it. Perhaps an O ring.

I reworked my gun today with the ball valve reversed.

A Schrader pilot cannon would be very efficient and could be configured to a smooth acting trigger.